The present invention pertains to the field of metal plating, and more specifically, to a method and multi-layer plating for improved corrosion resistance of articles having a ferrous metal substrate which provides satisfactory appearance for commercial utilization, perferably as either bright or dull chromium or chromium-substitute finished articles.
While various attempts have been made in the metal plating industry to develop chromium finished articles, or articles having the appearance of a chromium finish, from ferrous metal base stock which have suitable corrosion resistance to allow usage in corrosive environments, it has been discovered that the novel method and multi-layer plating of the present invention provides superior performance.
It has been known that ferrous metal articles could be plated with zinc or cadmium, followed by a layer of tin, in order to obtain a certain degree of corrosion resistance. U.S. Pat. No. 1,615,707 to Jones et al. is directed to such a plating sequence. However, there is no teaching or suggestion of combinations with a further layer of chromium or a chromium-substitute, as has been discovered can be advantageously utilized in accordance with the present invention. U.S. Pat. No. 3,011,957 to Eigemann likewise utilizes sequential platings of cadmium or zinc and tin, but such are indicated as being provided over a ferrous metal substrate which is first subjected to a phosphatizing treatment, in order to obtain improved corrosion resistance. Likewise, U.S. Pat. No. 3,011,957 sets forth no teaching or suggestion that a final layer of chromium or a chromium substitute could be advantageously utilized in combination with layers of zinc or cadmium or tin, as has been discovered in accordance with the present invention.
U.S. Pat. No. 3,997,301 to Yoshihara also indicates that sequentially plated layers of tin and zinc can be used over a low-carbon steel substrate. Yoshihara et al. further indicate that an optional film consisting of a hydrate of chromium oxide may optionally be superimposed over the zinc layer, in order to obtain improved corrosion resistance. However, Yoshihara et al., while disclosing that a hydrate of chromium oxide could be utilized, do not indicate or suggest that a layer of metallic chromium or a metallic chromium substitute could be used in such a combination for the top layer, as has been discovered in accordance with the present invention.
Thus, in view of the present state of the art, the novel combination of cadmium or zinc, tin and chromium or a metallic chromium substitute in accordance with the present invention has not previously been known or suggested. Consequently, the beneficial and advantageous results with regard to improved corrosion resistance which are provided in accordance with the present invention have not heretofore been known or available for use in the industry.